Ground fault circuit interruption (GFCI) devices have been sanctioned by the National Electric Code for use in residential circuits to protect against the hazards of electrical shock. Such GFCI devices, as presently commercially available, utilize a differential current transformer to sense a current imbalance in the line and neutral conductors occasioned by ground leakage current from the line conductor returning to the source through an unintended ground circuit path other than the neutral conductor. To prevent injurious electrical shock, the differential current transformer must develop a signal voltage of sufficient magnitude to enable a signal processor to initiate circuit interruption in the event of a current differential in the line and neutral conductors as low as 5 milliamps. For ease of manufacture and to provide a compact design, the line and neutral conductors, which constitute the primary windings of the differential current transformer, typically each make a single pass through the aperture of the toroidal transformer core. Thus to satisfy a 5 milliamp trip level, the signal processor must respond to a transformer primary excitation of less than 0.005 ampere-turns. To ease design constraints on the signal processor, the differential transformer should have a high permeability core and a secondary winding of many turns -- typically in excess of a thousand turns of very fine wire -- in order to develop signal voltages of manageable amplitudes. Signal levels are, nevertheless, quite low, 1 to 10 millivolts, requiring high amplification. With such high amplification, the processor design must insure amplifier stability and adequate noise immunity to prevent nuisance tripping.
An additional requirement of GFCI devices of this type necessary for Underwriters Laboratories listing is the capability of detecting a low impedance ground fault on the neutral conductor adjacent the load. Since the neutral conductor is also grounded at the source, such double grounding of the neutral conductor could create a situation where a portion of the ground fault current from the line conductor returns to the source through the neutral conductor. As a consequence, the current differential showing up in the differential transformer would not be truly indicative of the magnitude of the ground leakage current. It is thus seen that a low impedance neutral to ground fault has the potential of desensitizing the differential current sensor such that the GFCI would trip only in response to considerably higher ground leakage current levels. Under these circumstances, the GFCI device cannot afford protection to the degree intended.
Applicant's co-pending application, Ser. No. 571,930, filed Apr. 28, 1975, which is a continuation-in-part of application Ser. No. 509,462, filed Sept. 26, 1974, discloses an approach to GFCI sensor design which dramatically increases the fault signal amplitude while permitting utilization of a differential current transformer of less expensive construction. The disclosure of this co-pending application is specifically incorporated herein by reference. Pursuant to the approach disclosed therein, the differential transformer secondary is normally operated in a short circuited mode through an electronic switch. Current developed in the secondary winding in response to a current differential in the line and neutral conductors flows through the switch in shunt with a relatively high burden resistance. Periodically, the switch is momentarily opened to divert this secondary current through the burden resistance, developing thereacross a signal voltage of higher magnitude than can otherwise be achieved. A working embodiment of the invention disclosed in this co-pending application was found capable of developing signal voltage spikes of 200 millivolts peak amplitude in response to a 0.005 ampere differential in one-turn primary windings. With the prior art approach of continuously flowing secondary current through a burden resistor, signal voltages of 1 to 10 millivolts are typical. The differential current transformer embodied a ferrite core with a 125-turn secondary winding, as contrasted to an expensive, high permeability, nickel-iron core with on the order of 1500 secondary turns.
It is accordingly an object of the present invention to provide an improved ground fault circuit interrupting device of the type disclosed in the above-noted co-pending application having the capability of detecting desensitizing ground faults on the neutral conductor.
A further object is to provide a ground fault circuit interrupting device of the above character which is simple in design and inexpensive to manufacture.
Other objects of the invention will in part be obvious and in part appear hereinafter.